How do you start lilacs from existing ones?

Take semi ripe cuttings mid or late summer, for nodule cutting, just below a node. Remove side shoots, nodule cuttings should be 3/4 inches long. Dip base into rooting hormone, insert into standard cutting compost, if more than one cutting, ensure leaves do not touch. Place in propagating case at 21C. Shade from sun. Once rooted apply liquid fertilizer every two weeks